This invention relates to an optical magnetic-field sensor which is mainly used in a detection system for finding fault points at electric power transmission line networks, electric power-distribution line networks, transformer substations and the like, and more particularly to a method of producing such an optical magnetic-field sensor.
In order to automatically find fault points in electric power systems, optical magnetic-field sensors using optical single crystals (for example, BSO or the like) have been recently used. In this device, the light emitted from a light transmitter is transmitted through a magnetooptical element and received into a light receiver where the light is detected. If electric current values are rapidly changed due to short-circuit or ground-fault, the magnitude of a magnetic field produced around the power-transmission line is changed so that the polarization plane of the light transmitted through the magnetooptical element is changed. The change in polarization plane is detected to determine the occurrence of a fault.
Such an optical magnetic-field sensor includes a magnetooptical element, a polarizer and an analyzer which are built in the sensor. Further, optical fibers which are inserted into the sensor and have distal ends fixed to the sensor by means of ferrules. The collimation (forming bundles of parallel beams, and light condensing) of the light from the distal ends of the optical fibers is effected by rod lenses. In practice, cylindrical rod lenses are joined with and fixed to the distal ends of the ferrules, respectively, and other surfaces of the cylindrical rod lenses are opposed to the polarizer and the analyzer, respectively. The light transmitted through one of the optical fibers is fed through one of the rod lenses to the polarizer, and the light emitted from the analyzer is transmitted through the other rod lens onto the distal end of the other optical fiber.
In the optical magnetic-field sensor described above, however, it is required to adjust the optical axes of the distal ends of the optical fibers and the rod lenses. A rod lens has a refractive index distribution in radial directions formed by adjusting radial distributions of metallic atoms. Therefore, if the center of the optical fiber is not coincident with the center of the refractive index distribution, the light is not collimated efficiently. Moreover, as the center of refractive index cannot be determined by inspecting the external form of the rod lens, the adjustment or alignment of the optical axes is a two dimensional operation, which is a time-consuming and troublesome operation.
Moreover, in the event that rod lenses are used, optical axes of the distal end of the optical fiber and the rod lens are frequently not coincident with each other owing to the above reason. Accordingly collimation of light is incomplete resulting in increase in losses of quantity of light. Consequently, it has been impossible to enlarge distances between the light transmitter and the light receiver respectively and the fault detecting point to an extent beyond certain distances.